1. Field of the Invention
The invention pertains to a system and method for chemically coating a variety of surfaces with semiconductor materials, metals, or insulators for various applications Including electronics. More particularly, the invention pertains to methods for making chalcogenide compound films containing copper, for photovoltaic devices and other applications.
2. Description of Related Art
Numerous coating processes like electroless chemical, chemical vapor, and physical vapor depositions are commonly employed in industrial applications. Physical vapor deposition is commonly used in semiconductor manufacturing applications, often employing expensive vacuum techniques in order to sustain a relatively high film growth rate. Many such processes, while performed at high temperatures (e.g., greater than 300° C.), are non-equilibrium, often resulting in non-stoichiometric proportions. Also, due to the nature of the deposition processes, the deposited films often include relatively high defect densities. In the case of semiconducting devices, such high defect levels can limit electrical performance characteristics. In semiconductor device fabrication wherein p-n junctions are formed in a partial vacuum by depositing one film over a second film or a substrate of different conductivity type, the conventional evaporative and sputtering techniques may provide unsatisfactory film qualities. As an alternative, relatively more expensive techniques such as Chemical Vapor Deposition (CVD), Molecular Beam Epitaxy (MBE), pulsed laser deposition, and atomic layer epitaxy, are useful, especially with formation of III-V compound semiconductor materials, but satisfactory deposition processes have not been available for fabrication of thin film II-VI compound semiconductor materials.
Chacogenide compound films of CuAC or Cu(AB)C [where A is In, Al, Ga, Sn, Fe, Sb or any other transition metal; (AB) is (InGa), (InAl), (ZnSn) or (CdSn), or any combination of transition metals; and C is S or Se or the combination of SSe] are p-type semiconductor materials with great importance in solar cells and other optoelectronic applications. Bringing devices based on these materials to the market has been greatly impacted by the cumbersome and poor yield of their traditional growth techniques.
The traditional techniques of growing some of these materials include the following:
Three-stage batch co-evaporation of CuInGaSe on Mo-coated substrate as practiced by the National Renewable Energy Laboratory (NREL). The first stage is the deposition of (InGa)2Se3 layer at 400° C., and reacting it with Cu and Se at 550° C. during the second stage. The third stage, similar to the first stage, consists of the evaporation of In and Ga in the presence of Se at 400° C. [1,2].
CuInGaSe fabricated on Mo-coated glass by a hybrid co-evaporation/sputtering process as taught by Energy Photovoltaics, Inc. (EPV). In this process, In and Ga are first evaporated in the presence of Se vapor. The first layer is followed by sputtered Cu, and the film is selenized in Se vapor. In the final stage, In and Ga are once again evaporated in the presence of Se [1].
Co-evaporated CuInGaSe on Mo-coated stainless steel by Global Solar Energy, Inc. The Global Solar process is essentially three-stage, in that group III atoms (In and Ga) are deposited first, then Cu, followed by enough group III atoms to bring the film to its desired stoichiometry; each of these steps is done in the presence of selenium at high temperature. Deposition is performed onto continuously advancing 36 cm 300 m rolls of stainless steel foil at high deposition rates [1].
Shell Solar Industries (SSI) approach in the fabrication of CuInGaSSe involves sputtering a stacked precursor from alloyed Cu—Ga and In targets, then selenization in H2Se at elevated temperature, followed by sulfurization in H2S at elevated temperature. The depositions and reactions are performed on 3900 cm2 panes of soda-lime glass coated with a SiO2 diffusion barrier and Mo back contact [1].
Two-stage batch co-evaporated CuInGaSe on Mo-coated glass by the Institute for Energy Conversion (IEC). In this process, elemental Cu, In, Ga, and Se fluxes are independently controlled to provide a Cu-rich total flux, Cu/(In+Ga)>1, at the start of the run. Then, In, Ga, and Se fluxes only are applied until the desired final composition, Cu/(In +Ga)=0.8-0.9, is attained. The films are deposited at a 550° C. substrate temperature [1].
CuZnSnS deposited by inline-type vacuum apparatus. Here, ZnS, SnS and Cu are co-sputtered on a heated and rotated substrate in the vacuum chamber and then moved to reaction gas chamber for sulfurization using N2+H2S (20%) and annealing around 580° C. [3].
All the above are expensive vacuum techniques; and the process involved is based mostly on the intermixing of elements constituting the material and subjecting the mixture to high temperature to form the compound. Unfortunately each of the various elemental particles does not have the same surrounding. Hence, small isolated areas will have the right material composition after anneal leading to film with non-uniform stoichiometry. Thus, the process may be good for small area deposition but poor for large area deposition needed for high yield manufacturing.
Another method of growth adopted by Nanosolar is coating a homogeneously mixed ink of nanoparticles, in this case Cu, In, Ga, and Se, with industrial wet coating techniques followed by baking and sintering to form the CuInGaSe compound [4].
International Solar Electric Technologies (ISET)'s CuInGaSSe absorber is prepared by applying a mixed oxide precursor coating on a metallized glass substrate via a non-vacuum knife coating technique. The precursor coating is deposited using a water-based ink which contains nanoparticles of mixed oxides. After drying, the precursor ink is reduced under an atmosphere of H2 and N2 gas mixture to obtain a uniform and a smooth coating of Cu—In—Ga alloys. The resulting alloy coating is further selenized under an atmosphere of H2Se and H2S gases [1].
Both of the foregoing methods are non-trivial, as they involve nanoparticle growth, a process which is not as cheap as one expects, because the nanoparticle growth requires expensive chemicals and takes place at extremely slow rate. This is then followed by the expensive ink formulation process. Materials grown by this method will also suffer from non uniformity due to high probability of particles not having the correct surrounding particles throughout the growth area.
Copper indium disulfide (CuInS) thin films deposited via aerosol-assisted chemical vapor deposition using single source precursors. Growth at atmospheric pressure in a horizontal hot-wall reactor at 395° C. yielded best device films as claimed by the author. Post-deposition sulfur-vapor annealing enhanced stoichiometry and crystallinity of the films. However, the single precursor is a very expensive organometallic (PPh3)2Cu(SEt)2In(SEt)2. The high precursor cost may not be compatible with the low cost devices like solar cells [5].
CuSbS was also deposited by traditional chemical bath deposition (CBD). This involves growing SbS first followed by CuS. A 6700 Å film took about 7 hours to grow. This slow growth rate and the accompanying waste make this process unacceptable [6].
CuInS2 was equally deposited by spray pyrolysis [7]. Here they sprayed aqueous solution of 0.01M of CuCl2.2H2O, InCl3, and CS(NH2)2 in a 1:1:2 (by volume) onto substrates at various temperatures of 225, 250 and 275° C. CuSbS2 films were also obtained by Spray Pyrolysis Deposition [8]. Here the precursor weight ratio (CuCl2.2H2O:H2NCSNH2:(CH3COO)3Sb) was varied between 2.57:1:5.71 and 6.86:1:5.71, at 240° C. Most of the time the film morphology is less than desirable and the electrical properties of these films are impaired by non-volatile unwanted elements left behind in the films; the films are therefore of less practical importance.
Additional background information may be found in the following references, whose numbers correspond to the respective citations in the foregoing discussion:    1. I. L. Repins et al. Comparison of device performance and measured transport parameters in widely-varying Cu(In,Ga)(Se,S) solar cells. Prog. Photovolt: Res. Appl. 14:25-43, 2006.    2. J. Ward et al. Cu(In,Ga)Se2 Thin-film concentrator solar cells. NCPV Prog. Rev. Meeting Lakewood, Colo., Oct. 14-17, 2001. NREL/CP-520-31144.    3. K. Jimbo et al. Cu2ZnSnS4-type thin film solar cells using abundant materials. Thin Solid Films 515: 5997-9, 2007.    4. Nanosolar Inc.; High-performance thin-film photovoltaics using low-cost process technology. 17th Int'l Photovoltaic Sci. Eng. Conf., Tokyo, Japan, Dec. 3-7, 2007.    5. A. F. Hepp et al. Aerosol-assisted chemical vapor deposited thin films for space photovoltaics; NASA/TM-2006-214445.    6. S. Messina et al. Antimony sulfide thin films in chemically deposited thin film photovoltaic cells. Thin Solid Films 515:5777-82, 2007.    7. S. Aksay, “Structural and morphological properties of CuInS2 polycrystalline films obtained by spray pyrolysis method. J. Arts and Sci. 4:1-9, 2005.    8. S. Manolache et al. The influence of the precursor concentration on CuSbS2 thin films deposited from aqueous solutions. Thin Solid Films 515:5957-60, 2007.